crataegus tomentosa

from WordNet (r) 3.0 (2006)

Crataegus tomentosa
    n 1: erect and almost thornless American hawthorn with somewhat
         pear-shaped berries [syn: {blackthorn}, {pear haw}, {pear
         hawthorn}, {Crataegus calpodendron}, {Crataegus tomentosa}]

from The Collaborative International Dictionary of English v.0.48

Blackthorn \Black"thorn`\, n. (Bot.)
      (a) A spreading thorny shrub or small tree ({Prunus
          spinosa}), with blackish bark, and bearing little
          black plums, which are called sloes; the sloe.
      (b) A species of {Crat[ae]gus} or hawthorn ({Crat[ae]gus
          tomentosa}). Both are used for hedges.
          [1913 Webster]